1. Field of the Invention
The present invention relates generally to image forming apparatuses, and more particularly, to an image forming apparatus permitting a toner image formed on an image holding member to be transferred onto a recording medium by a transfer roller.
2. Description of the Related Art
FIG. 7 is a perspective view showing an over view of a conventional full-color laser beam printer 5 as seen from its front side, and FIG. 8 is a view showing the structure of the internal mechanism of full-color laser beam printer 5.
Referring to FIG. 8, printer 5 includes a photoreceptor unit 10 having a photoreceptor drum 11 driven to rotate in the direction of arrow a, a laser scanning optical system 20, a full-color developing unit 30, an intermediate transfer unit 40 including an endless intermediate transfer belt 41 driven to rotate in the direction of arrow b, and a paper feed unit 60.
A charging brush 13 and a cleaner 12 are provided around photoreceptor drum 11. Charger brush 13 charges the surface of photoreceptor drum 11 uniformly to a prescribed potential. Cleaner 12 takes away toner remaining on photoreceptor drum 11 by its blade 12a.
Laser scanning optical system 20 is of a known type including a laser diode, a polygon mirror 21, and an f.theta. optical device therein, and image data for each of C (cyan), M (magenta), Y (yellow), and Bk (black) is transferred to the control unit of the system from the host computer. Polygon mirror 21 is driven to rotate by a polygon mirror motor M2. Laser scanning optical system 20 sequentially outputs image data for every color as a laser beam, and exposes photoreceptor drum 11 by scanning. Electrostatic latent images in the colors are sequentially formed on photoreceptor drum 11.
Full-color developing unit 30 is formed by integrally attaching four color developers 31C, 31M, 31Y and 31Bk storing developing agents containing toner of C, M, Y, and Bk, respectively to a developer rack (not shown) which can rotate clockwise around the pivotal axis. Each developer is switched as it rotates every time an electrostatic latent image in each color is formed on photoreceptor drum 11 such that the developing sleeve 32 of a corresponding developer is positioned at a developing position D. Printer 5 uses the rotary type full-color developing unit 30 to make the entire printer compact.
The intermediate transfer belt 41 of intermediate transfer unit 40 is stretched in an endless manner around driving roller 42, support roller 43 and tension rollers 44 and 45, and driven to rotate in the direction of arrow b in synchronization with photoreceptor drum 11. Driving roller 42 and photoreceptor drum 11 are both driven to rotate by a main motor M1. There is provided a single belt mark (not shown) at a side edge of intermediate transfer belt 41 excluding the image region for the purpose of registration of tip ends of color images, and a belt position detection sensor SE1 to detect the position of the belt by detecting the belt mark is provided close to intermediate transfer belt 41.
Intermediate transfer belt 41 is in contact with photoreceptor drum 11 under pressure by a rotatable primary transfer roller 46. Intermediate transfer belt 41 faces a horizontal transport path 65 for recording paper, which will be described, at the portion supported by support roller 43 and is in contact with a rotatable secondary transfer roller 59.
Primary and secondary transfer rollers 46 and 59 are each provided with a known voltage supply unit to supply bias voltage of an opposite polarity to the charging polarity of toner. Primary transfer roller 46 allows a toner image on photoreceptor drum 11 to be transferred onto intermediate transfer belt 41, while secondary transfer roller 59 allows the toner image on intermediate transfer belt 41 to be transferred onto a recording paper sheet.
Cleaner 50 is provided in the space between developing unit 30 and intermediate transfer belt 41. Cleaner 50 has a cleaner blade 51 to remove toner remaining on intermediate transfer belt 1. Blade 51 and secondary transfer roller 59 are provided in the way they can be freely abutted against or detached, in other words in a contactable/separable manner from intermediate transfer belt 41.
Paper feed unit 60 includes a manual feed tray 61 which can be opened/closed on the front side of printer main body 5 (the side on which an operator usually stands), a feed cassette 64 loaded in main body 5 which can be replaced from the front side, a feed roller 62 and a timing roller 63. A stack of recording paper sheets placed on feed tray 61 or recording paper sheets S accommodated in feed cassette 64 in a layered manner are fed to the right in FIG. 8 on a one-sheet basis by the rotation of feed roller 62, synchronized with images formed on intermediate transfer belt 41 by timing roller 63, and sent onto a secondary transfer zone. Horizontal transport path 65 for storing paper sheets S is formed of an air suction belt 66, for example, and there is provided a vertical transport path 71 including transport rollers 72, 73, and 74 from a fixing device 70. Recording paper sheets S are discharged onto the upper surface of printer main body from this vertical transport path 71.
Now, the operation of printer 55 will be described. At the beginning of the printing operation, secondary transfer roller 59 and blade 51 are apart from intermediate transfer belt 41. As the printing operation is initiated, main motor M1 is driven and rotates a photoreceptor drum in the direction of arrow a and intermediate transfer belt 41 in the direction of arrow b at the same circumferential speed. Photoreceptor drum 11 is charged up to a prescribed potential by charging brush 13.
Then, a cyan image is exposed by laser scanning optical system 20, and the electrostatic latent image of the cyan image is formed on photoreceptor drum 11. The electrostatic latent image is immediately developed by developer 31C, and the toner image is transferred onto intermediate transfer belt 41 by a primary transfer zone. Immediately after the primary transfer operation, developer 31M is switched to developing position D, followed by exposure of a magenta image, development thereof and a primary transfer thereof. Similarly, switching to developer 31Y, exposure of a yellow image, development thereof, and a primary transfer thereof follow. Further, switching to developer 31Bk, exposure of a black image, development thereof and a primary transfer thereof are performed to place toner images one after another on intermediate transfer belt 41 for every primary transfer.
After the last primary transfer operation completes, developing unit 30 is switched to developer 31C for the next printing processing, while secondary transfer roller 59 and blade 51 are pressed-contacted to intermediate transfer belt 41. At the time recording paper sheets S are sent into the secondary transfer unit, and a full-color toner image formed on intermediate transfer belt 41 is transferred onto a recording paper sheet S. After the secondary transfer operation, secondary transfer roller 59 and blade 51 are detached from intermediate transfer belt 41.
Recording paper sheet S transferred with the full-color toner image is transported to fixing device 70 by air suction belt 66 for fixation, and discharged onto the upper surface of printer main body 5 by transport rollers 72, 73 and 74.
In such conventional printer 5, a skin-coated roller having a smooth skin layer formed on its surface is used for each of transfer roller 46 and 59. The skin-coated roller which can be reduced to only a limited level in hardness imposes high transfer pressure, which causes void in a solid area of printed characters.
In order to solve the problem of the skin-coated roller, use of a cellular (foamed) roller having a surface formed of a cellular material which can be readily reduced in hardness has been proposed. When used as transfer rollers 46 and 59, however, the cellular roller causes a differential between the transfer efficiencies of the paper passed portion through which a recording paper sheet S of a small width has passed and the non-paper passed portion through which no paper sheet has passed when the width of recording paper sheet S is switched from a small size to a large size, resulting in density variations in images (size memory).